What Does P0408 Mean?
The EGR (Exhaust Gas Recirculation) position sensor B circuit is reporting a voltage above the expected maximum threshold. The ECM detects the sensor B signal is stuck high, suggesting an open ground, short to voltage, or internal sensor failure. The driver may notice rough idle if the ECM interprets the high reading incorrectly, or no symptoms at all if the system defaults to sensor A for control.
Common Causes
30%
Failed EGR position sensor B with high voltage output
25%
Signal wire shorted to 5V reference or battery voltage
20%
Open ground circuit in the sensor B circuit
15%
Connector corrosion or damage causing high-resistance ground path
10%
ECM internal failure on the sensor B input
Diagnostic Steps
1
Check the EGR sensor B voltage PID with key on engine off — should read approximately 0.4–0.9V. If reading near or above 4.5V, confirm the high circuit condition.
2
Disconnect sensor B connector and measure the signal line voltage at the ECM side — if it drops near 0V, the sensor is internally failed. If it stays high, there is a wiring fault.
3
Test the ground circuit continuity from the sensor connector ground pin back to the ECM ground pin — should be under 5 ohms. An open ground causes the signal to float high.
4
Inspect the wiring for chafing against power wires or hot exhaust components that could damage insulation and cause a short to voltage.
5
Replace the EGR sensor B (or EGR valve assembly if the sensor is not separately serviceable) if wiring and connector checks pass.
Estimated Repair Cost
$80 - $350
Parts + labor, varies by vehicle and location
The EGR (Exhaust Gas Recirculation) position sensor B circuit is reporting a voltage above the expected maximum threshold. The ECM detects the sensor B signal is stuck high, suggesting an open ground, short to voltage, or internal sensor failure. The driver may notice rough idle if the ECM interpret...
The most common cause of P0408 (Exhaust Gas Recirculation Sensor B Circuit High) is: Failed EGR position sensor B with high voltage output
Typical repair costs for P0408 range from $80 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to drive. This is a secondary sensor fault that primarily affects EGR monitoring accuracy. The vehicle will fail emissions testing. No immediate safety concerns.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0408 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions Control / EGR System
Difficulty
Type
Generic (SAE)
Recommended Tools
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